A high lift system of an aircraft serves the purpose of lift and drag management. A high lift system is often composed of a leading edge slat system and a trailing edge flap system. Many flap systems in civil and military aircraft are equipped with a central drive unit, which is also known as power control unit (PCU), which drives a transmission shaft train and local mechanical actuator devices, the so-called drive stations, on corresponding support stations of the movable surfaces. The high lift settings are selectable by a cockpit crew through a flaps lever, through which a surface angle is selectable.
Such a transmission system provides a load path from the central drive unit to all actuator outputs, leading to a symmetrical deployment of all high lift surfaces. Certain kinematics in the drive stations transform a rotary motion driven by the drive station into a required surface movement.
A high lift flap system is often controlled and monitored by control computers, the so-called flap channel of the slat flap control computers (SFCC). System drive commands primarily originate from an input device, such as a flaps lever input. The surfaces will be driven to predetermined positions (flap or slat settings) that are laid down in the software of the respective control computer. For achieving a high accuracy in driving the high lift surfaces to the predetermined positions, positions are continuously fed back/monitored by feedback position pick-off units (FPPU) attached to the drive unit and fitted with an internal gearbox to dedicate an equivalent system angle.
Further sensors are dedicated to system failure monitoring such as station position pick-off units (SPPU), which are connected to individual drive stations to dedicate an equivalent angle for each station for system monitoring purposes.
Flap or slat attachment monitoring is useful for detecting a potentially abnormal state of a driven flap or slat. Commonly, each flap or slat is driven by two stations and the position of these two stations are monitored by two independent station position pick-off units. The above-mentioned control computer may be provided with a flap or slat skew monitoring for detecting an abnormal surface twist (skew). In case a predetermined skew threshold is exceeded, the control computer may interrupt the operation of the high lift system.